This invention relates in general to the field of mobile terminals, such as cellular telephones, and more particularly relates to multi-mode, dual band mobile terminals.
One conventional dual band, multi-mode mobile terminal contains two receivers, one for GSM1900 (Global System for Mobile Communications (GSM), 1900 MHz) service and one for AMPS (Advanced Mobile Phone Service (AMPS), 800 MHz) service. In addition to operating in different frequency bands, these system also typically exhibit different channel bandwidths and spacings. For example, TDMA1900 and AMPS channels have a width of 30 kHz, while GSM1900 employs a 200 kHz channel width.
Recently some network providers have expressed an interest providing a Digital AMPS (DAMPS) plus GSM1900 mobile terminal. As can be appreciated, due to cost and size constraints it would be very desirable to provide but a single receiver in the mobile terminal, as opposed to two separate receivers (one for each band of interest).
A single receiver mobile terminal would normally contain only one crystal oscillator, and as a result only one system-specific baseband (BB) clock can be derived at any given time (typically by using a phase locked loop (PLL)). If the mobile terminal happens to be camped on a low priority network, it must search periodically search for a higher priority network. Since, for example, the GSM1900 and DAMPS networks cannot communicate their presence to the mobile terminal, the mobile terminal must be able to search for the higher priority network itself. However, the mobile terminal does not know a priori which channels are TDMA1900, CDMA1900 or GSM1900 channels. If the mobile terminal must then periodically change its BB clock frequency when searching for other (higher priority) channels, then synchronization is lost to the current channel on which the mobile station is camped. Since this channel will normally be a channel on which the mobile terminal receives pages, which inform the mobile terminal of the presence of an incoming call, it is possible that the mobile terminal can miss incoming calls when searching for other networks. This is obviously an undesirable situation from both the user""s and the network operator""s point of view.
It is noted that one might consider using a Fast Fourier Transform (FFT) frequency spectrum analysis or filtering method to search for a desired carrier. However, this method is not desirable since the location of the 30 kHz carriers can vary within the 200 kHz window.
It is a first object and advantage of this invention to provide a technique that enables a mobile terminal to identify and distinguish transmissions from different wireless services, without having to retune a baseband clock that is used to receive pages.
It is another object and advantage of this invention to provide a multi-mode, dual band mobile terminal having an ability to recognize and distinguish, by example, a GSM1900 channel from a TDMA1900 channel or a CDMA1900 channel, without having to use the GSM1900-specific baseband clock, thereby reducing unnecessary baseband clock changes and a number of missed pages from a currently serving system.
It is a further object and advantage of this invention to provide a multi-mode, dual band mobile terminal having an ability to recognize and distinguish, by example, a CDMA800 carrier from a TDMA800/AMPS carrier.
The foregoing and other problems are overcome and the objects and advantages of the invention are realized by methods and apparatus in accordance with embodiments of this invention.
This invention teaches a technique to identify and distinguish, for example, a GSM1900 carrier from a TDMA1900 or a CDMA1900 carrier, without requiring a change to the system-specific BB clock frequency. The use of this invention thus aids in avoiding unnecessary BB clock frequency changes. The mobile terminal constructed and operated in accordance with this invention accomplishes this important function by selectively filtering and then measuring received signal levels.
A specific method is disclosed for identifying a GSM1900 channel with an AMPS/TDMA800/TDMA1900/GSM1900 dual band, quadruple mode mobile terminal when operating in the 800 MHz band in the AMPS or the TDMA800 mode. In this method the mobile terminal measures Received Signal Strength Indicator (RSSI) levels in a candidate GSM1900 MHz channel through a 30 kHz bandpass filter, and a 200 kHz bandwidth GSM1900 channel is identified if the RSSI values of at least three adjacent 30 kHz channel measurements are sufficiently close together.
An underlying assumption made by the teachings of this invention is that, in accordance with typical network design practice, adjacent 30 kHz TDMA1900 channels are not used in the same cell. As such, if the mobile terminal finds sufficiently large RSSI values in at least three adjacent 30 kHz channels, the presence of TDMA1900 channels is unlikely, and instead a wider bandwidth 200 kHz GSM1900 channel may be present.
An important benefit that results from the practice of the teachings of this invention is that the mobile terminal is enabled to recognize and distinguish a GSM1900 carrier from a TDMA1900 or a CDMA1900 carrier, without requiring a change in the system-specific baseband clock frequency. Similarly, a TDMA800/AMPS carrier can be distinguished from a CDMA800 carrier (1.23 MHz bandwidth). By the use of this method the mobile terminal is enabled to reduce unnecessary baseband clock frequency changes, and thus reduces the chance that a page will be missed during a search for another network.
The method in accordance with this invention employs a technique based on a frequency spectrum analysis, and exploits to advantage the fact that the GSM1900 channel width is 200 kHz, that the TDMA800/AMPS/TDMA1900 channel width is 30 kHz, and that the CDMA800 channel width is 1.23 MHz.
This invention thus provide a method for use by a multi-mode, dual band mobile terminal for identifying a presence of a GSM1900 carrier that is channelized into first channels having a bandwidth of 200 kHz, and for distinguishing the GSM1900 carrier from at least a TDMA1900 carrier that is channelized into second channels having a bandwidth of 30 kHz. The method includes steps of (a) tuning a receiver to a frequency of a candidate GSM1900 channel; (b) determining a value of a received signal strength indicator (RSSI) using a 200 kHz receiver passband filter; (c) if the RSSI value is above a predetermined threshold, converting the frequency of the candidate GSM1900 channel into a frequency of a TDMA1900 channel using a predetermined relationship; (d) retuning the receiver to the frequency of the TDMA1900 channel; (e) determining a RSSI value using a 30 kHz receiver passband filter for the TDMA1900 channel, and also for at least two adjacent TDMA1900 channels, assuming that two adjacent TDMA1900 channels are not used together in a same cell; and, if the at least three RSSI values are all found to be approximately equal within some threshold value, (f) making at least an initial assumption of the presence of the GSM1900 carrier. A further step can be executed of verifying that the assumed GSM1900 carrier is not actually a CDMA1900 carrier.
The method has an initial step of camping on a channel of a carrier in another frequency band (e.g., a TDMA800 channel), and the steps of tuning and retuning are executed without changing a baseband clock frequency from a frequency associated with the channel that the mobile terminal is camped on.